The authors thank D. Twitchen and M. Markham from Element Six for providing diamond samples, and C.L. Yu, P. Hemmer and O. Bakr for helpful discussions. The authors also thank K.P. Chen and V. Shalaev for their helpful suggestions. T.M.B. acknowledges support from the National Defense Science and Engineering Graduate (NDSEG) and National Science Foundation (NSF) Graduate Research fellowships, and J.T.C. acknowledges support from the NSF Graduate Research fellowship. Devices were fabricated in the Center for Nanoscale Systems (CNS) at Harvard. This work was supported in part by Harvard University's Nanoscale Science and Engineering Center (NSEC), a NSF Nanotechnology and Interdisciplinary Research Team grant (ECCS-0708905), the Defense Advanced Research Projects Agency (Quantum Entanglement Science and Technology program), and the King Abdullah University of Science and Technology Faculty Initiated Collaboration Award (FIC/2010/02).